Double needle, decorative and functional stitching is often desirable to use for such apparel items as shirt pockets and pocket flaps. The flaps normally utilize the two needle stitching as purely decorative since the stitching does not serve a primarily functional purpose, whereas two needle stitching on pockets is both decorative and functional. When two needle decorative stitching is required, a special type of sewing machine is employed to produce the desired "mitered corner" decorative effect at pivot points. This machine type employs two needles to simultaneously sew parallel seams. The machines, which are referred to as "split needle bar" machines, differ from ordinary double needle machines in that the left or right needle can be selectively disabled in order to produce the mitered corner effect at pivot points. The distance between the needles is referred to as the needle gauge and the machines are normally available with gauges of 1/16th inch to 1/2 inch in 1/16th inch increments.
Although split needle bar machines produce a very pleasing decorative effect, operation of the machine requires considerable skill and time to selectively disengage and re-engage the appropriate needle at each pivot point. If desired, a two needle decorative stitching pattern can be achieved by using a conventional single needle machine. When a single needle machine is employed, the operator is required to stitch the decorative pattern two times utilizing a different seam margin each time. This technique is very time consuming since the operation must be sewn twice and the machine must be adjusted to provide a different seam margin for the second seam. In addition, the stitches in the two seams are not "synchronized" such that the individual stitches in the double needle mode may not be "in step" with each other. This lack of synchronization degrades the appearance of the two needle decorative stitching.
Conventional double needle machines cannot be used to perform the two needle decorative stitching operation because the inside needle must not sew as many stitches as the outside needle if the correct mitered corner is to be obtained at a pivot point. This is due to the fact that the additional stitches sewn generate what is referred to in the apparel industry as the "crow's foot" which is not acceptable on quality garments.
Split needle bar machines which have the ability to selectively disable the left or right needle have been developed to solve the two needle decorative stitching problem. These machines are available from a wide variety of sewing machine manufacturers and include among others the Brother Model 835, Durkopp Model 380, Juki Model 528, Consew Model 328 or Pfaff Models 542 or 1242. These machines have mechanical linkages which can be manually operated by the operator to disengage the desired needle. Since most two needle decorative stitching is performed as a top stitching or setting operation, the operator must precisely control the distance from the outside edge of the material to the outside needle. This distance is referred to as the "seam margin". In order to sew a 90.degree. mitered corner correctly, the operator sews along the first segment until a point is reached that is equal to the sum of the distance between the two needles and the seam margin which is defined as the pivot point of the mitered corner. At that point, referred to hereinafter as the "pivot point", additional stitches would be sewn using the right needle until the outside corner of the mitered corner was reached, referred to hereinafter as the "apex" of the mitered corner. At the apex, the presser foot would then be lifted and the material pivoted in a counterclockwise direction in preparation for sewing the second seam, for example, 90.degree. for a right angled mitered corner. The presser foot would then be lowered and additional stitches would be sewn with the right needle until the needle is again lined up with the pivot point. The left needle is then re-engaged at the pivot point and sewing would resume to sew the second seam.
In practice, the operator normally has to shorten or lengthen the last stitch in the double needle mode in order to achieve the correct pivot point. This may also be the case with the stitches sewn from the pivot point to the apex. For example, if the angle to be turned is 90.degree. and if the needle gauge is 1/4 inch, a stitch density of 8 stitches per inch (spi) will assure that two stitches with the right needle before and after the apex will provide the correct mitered corner. Although this works quite well with a 90.degree. corner, a mitered corner in the same sewing operation may involve a cornering angle of 60.degree.. This case represents a "complex" case in that the correct mitered corner cannot be obtained unless the length of one single needle stitch before the apex and one single needle stitch after the apex is varied from the standard stitch length (1/8th of an inch at 8 spi in this example). This is also the situation with cornering angles for 30.degree., 120.degree. and 150.degree.. A different number of complete and/or partial stitches is required to achieve the correct mitered corner in each of these cases.
In order to manually sew a mitered corner, the operator produces shorter (or longer) stitches by overriding the sewing machine's feeding machanism during the cornering sequence. The operator must calculate how many full and partial stitches are needed for each cornering angle in an empirical "trial and error" manner and thus repeat the required sequence consistently on subsequent parts. The distance from the edge at which the cornering sequence must be initiated varies according to the cornering angle. This requires the operator to make a number of decisions such as determining the distance from the edge for initiating the cornering sequence, adjusting the length of the last stitch sewn in the double needle mode, disengaging the appropriate needle, determining the accurate dimensions for the seam margin, adjusting the length of the last stitch, realigning the material, sewing the required number of stitches with the single needle and re-engaging the disabled needle to resume double needle sewing. These steps require a high level of operator skill to perform the cornering sequence correctly. In addition, a considerable amount of time is required to produce the correct mitered corner at a pivot point because of the manipulation of the sewing machine required to disengage the needle and sew the normal and partial stitches.
Sewing machine manufacturers have developed control mechanisms that simplify the operation of the "simple" cases. For example, the Juki Model ACl and the Brother Model 835-903 control units can be used to program a "simple" cornering sequence involving only normal length stitches. In addition, an apparatus for split bar needle sewing of mitered corners is illustrated in U.S. Pat. No. 4,359,953 issued to the present applicant. With these units, the operator must sew to the pivot point and disengage the appropriate needle. The control unit will then cause a programmed number of stitches to be sewn with the single needle and then stop with the needle down. The operator must then lift the presser foot, pivot and realign the material and then depress the foot treadle. The control unit will cause a programmed number of stitches to be sewn in the single needle mode and then will automatically re-engage the needle. These control units are helpful but they still require considerable operator skill and they cannot handle the "complex" cases.
In view of the above described disadvantages with the prior art, there exists a need for a sewing machine for sewing mitered corners which automatically controls the operation of the sewing machine. In so doing, a higher degree of repeatability and quality will be attained without having to maintain a high level of operator skill at the machine.